Three-way catalysts, so-called because they can simultaneously carry out the oxidation of carbon monoxide (CO) and hydrocarbons (HC) and the reduction of nitrogen oxides (NOx), are used for efficiently eliminating noxious components such as CO, HC and NOx present in the exhaust gases of internal combustion engines such as automotive engines. Catalysts used as such three-way catalysts are composed of noble metals belonging to the platinum group (PGM), such as platinum (Pt), rhodium (Rh) and palladium (Pd), that are supported on a porous carrier made of a metal oxide such as alumina (Al2O3). Three-way catalysts containing a plurality of such PGM noble metals are able to exhibit an especially high exhaust gas purifying catalyst function on the exhaust gases generated from burning, within an internal combustion engine, a mixed gas near the stoichiometric air-fuel ratio (also abbreviated as “stoich”:A/F=14.7).
However, continuing to maintain the air-fuel ratio of the mixed gas supplied when actually using an internal combustion engine (typically when operating an automobile) at a near-stoichiometric ratio is difficult. That is, for example, depending on the running conditions of the automobile, the air-fuel ratio of the mixed gases may have excess fuel (referred to as “rich”: A/F<14.7), or may have excess oxygen (referred to as “lean”: A/F>14.7). It has become common recently to include inorganic materials having an oxygen storage capacity (OSC), also referred to as “OSC materials,” within catalyst carriers.
In three-way catalysts, composite oxides composed primarily of ceria (CeO2) and zirconia (ZrO2) (also referred to below as “CZ composite oxides”) have been used to date as OSC materials. For example, Patent Document 1 discloses an example of a conventional exhaust gas-purifying catalyst provided with an OSC material made of a CZ composite oxide for which the solid solubility of zirconium oxide in cerium oxide is 50% or more and which is characterized in that the crystallites making up the particles of CZ composite oxide have an average diameter of 100 nm or less. Patent Document 2 teaches a method of producing particles of a CZ composite oxide used as an OSC material, which particles have a crystallite diameter of about 10 nm.